In the related art, a light source that outputs incoherent light (hereinafter often referred to as an incoherent light source), such as a super luminescent diode (SLD), is used, as a light source for an electoro-optical probing (EOP) device, an electro-optical frequency mapping (EOFM) device, and a magneto-optical frequency mapping (MOFM) device regarding fault analysis of a semiconductor or the like (for example, see Patent Literature 1). Such incoherent light has a merit that it is more difficult for interference fringes to be generated as compared with laser light. On the other hand, an incoherent light source is generally inferior to a laser light source in terms of an SN ratio.
For example, Non-Patent Literature 1 discloses a light source device in which an SLD that is an incoherent light source and a semiconductor optical amplifier (SOA) that is an optical amplifier are combined. In the semiconductor optical amplifier, when the power of input light increases, carriers stored in an active layer are consumed in order to amplify the input light, and a gain is decreased. In a state in which the gain is decreased (gain saturation), light amplification characteristics of the semiconductor optical amplifier becomes nonlinear. By amplifying the input light in a region in which such amplification characteristics are nonlinear, it is possible to suppress the intensity of noise in the input light.